1. Field of the Invention
This invention relates to a novel electrical insulating oil composition. More particularly, the invention relates to an electrical insulating oil composition which is excellent in low temperature characteristics and is suitable for use in oil-filled capacitors.
2. Description of the Prior Art
In the conventional art, PCB (polychlorobiphenyl) was widely used as an insulating oil for high power capacitors. PCB has a high dielectric constant, however, the use of PCB was prohibited because its toxicity was found. After that, in order to provide insulating oils having a high dielectric constant, there have been proposed insulating oils comprising a mixture of chlorinated alkyldiphenyl ether, phthalic acid esters and benzene trichloride; and esters of benzyl alcohol and fatty acids.
As PCB has a high dielectric constant, a solid insulating material of insulating paper or combined films of insulating paper and biaxially oriented polypropylene film was used for capacitors. However, as the power loss of PCB and paper is large, the power loss of capacitors with these materials as the whole was large, especially at lower temperatures. For example, the loss at temperatures of +10.degree. to +20.degree. C. is approximately 0.1%, meanwhile the loss is abruptly increased by ten times to 1% at temperatures of -20.degree. C. to -30.degree. C. For this reason, the generation of heat by the power loss in a capacitor cannot be disregarded and the temperature rise of 20.degree. C. to 30.degree. C. is caused to occur which will further depend upon the sizes of capacitors, kinds of of solid insulating materials and configurations of electrodes. As a result, even when the temperature of an insulating oil is low, for example below the pour point, the temperature is gradually raised by the internal heat generation of the capacitor. The temperature thus exceeds the pour point of the insulating oil in due course, and finally, the viscosity is lowered and the insulating oil can act as a liquid substantially. As a result, PCB was regarded that it can be used under considerably low temperature conditions. In other words, the heat generation by power loss is essentially undesirable, however, it was exceptionally regarded desirable in the case of PCB in low temperature uses.
Meanwhile, bicyclic aromatic hydrocarbons such as 1-phenyl-1-xylylethane (PXE) and monoisopropylbiphenyl (MIPB) were proposed as the substitute for PCB. The power loss of them is small as compared with that of PCB. The loss is on the level of about 0.01% to 0.02% which is one tenth of PCB capacitor. Even at temperatures as low as -40.degree. C., the dielectric loss does not exceed 0.1%. Accordingly, the temperature rise in a capacitor owing to the power loss is generally lower than 5.degree. C. In the case of capacitors impregnated with the bicyclic aromatic hydrocarbons, the compensation by the self heat generation of power loss in lower temperatures like PCB capacitors cannot be expected.
The insulating oils of the series of the foregoing bicyclic aromatic hydrocarbons are excellent in the partial discharge characteristics as compared with PCB and the like compounds having a high dielectric constant. In addition, the former ones are excellent also in impregnating property relative to solid insulating materials such as plastic films. Accordingly, they are mainly used for electric capacitors.
For the above reason, it has been eagerly desired to propose bicyclic aromatic hydrocarbons that are useful in lower temperatures with making the most of the advantages of the bicyclic aromatic hydrocarbons.
The insulating oils of bicyclic aromatic hydrocarbons that are used at present are the foregoing PXE and MIPB; and the mixture of monobenzyltoluene (MBT) and dibenzyltoluene (DBT). Any of these substances has a low temperature characteristic that is superior to that of PCB. In order to improve further the adaptability and the partial discharge characteristic at lower temperatures, the inventors of the present application has made detailed investigation with regard to the relation between the structures of bicyclic aromatic hydrocarbons and the properties of them as electrical insulating oils.
In the first place, alkyl groups having 1 to 5 carbon atoms were added to the skeletal carbon chains of 1,1-diphenylethane so as to synthesize the model compounds of the basic skeletal structure of bicyclic aromatic hydrocarbons. The properties as synthetic oils were investigated with regard to six kinds of synthetic oils including the compound having only the basic skeletal structure.
The structures of the synthetic oils are represented by the following structural formula: ##STR1## wherein R is a methyl group, dimethyl group, and ethyl group; isopropyl group, tert-butyl group, and tert-amyl group.
Each of the synthetic oils was refined by clay treatment to make the dielectric loss tangent below 0.02% at 80.degree. C., which was used by several kinds of tests as insulating oils for capacitors. In order to observe the basic properties as insulating oils, hydrogen gas absorbing capacity was measured, the results of which are shown in FIG. 1. According to these results, the hydrogen gas absorbing capacity increases with the decrease of the number of carbon atoms in substituent groups, i.e., with the rise of aromaticity (the percentage of aromatic carbons in the total structure). Taking the above fact into consideration, all-film type model capacitors were made by using the respective synthetic oils and their performance was tested.
Two sheets 14 micrometers thick biaxially oriented polypropylene films were put together in layers. With using the thus prepared films as insulating materials, aluminum foil 7 micrometers thick was wound to obtain capacitors of 0.3 to 0.4 .mu.F.
Breakdown voltages were measured by applying electric voltages to these capacitors in a room at a temperature of 25.degree..+-.3.degree. C. An electric voltage (2400 V) which corresponds to 50 V/.mu. in potential gradient was applied to the capacitors for 24 hours and after that, the electric voltage was raised by 10 V/.mu. at an interval of 48 hours. The number of capacitors was 6 for each synthetic oil and the times at which capacitors were broken down were recorded and their average was taken as the value of each group of capacitors.
The results obtained in the above tests are shown in FIG. 2. According to these results, the voltage withstanding characteristics become higher with the rise of aromaticities of the compounds, that is the lowering of molecular weights, which correspond to the tendency of hydrogen gas absorbing capacities of the compounds shown in FIG. 1.
It was understood from the results shown in FIG. 1 and FIG. 2 that the hydrogen gas absorbing capacity and the voltage withstanding characteristic become better with the lowering of the molecular weights of bicyclic aromatic hydrocarbons.
The viscosity becomes low with the lowering of molecular weight of bicyclic aromatic hydrocarbon, however, the melting point becomes high because the compound structure is simplified, which fact makes the low temperature characteristics worse.
The melting points of bicyclic aromatic hydrocarbons (non-condensed type) having 12 carbon atoms such as biphenyl and those having 13 carbon atoms, next to the lowest number of carbon atoms, such as methylbiphenyls (3 kinds of isomers of 2-methylbiphenyl, 3-methylbiphenyl and 4-methylbiphenyl) are high (about 0.degree. C. to 69.1.degree. C.) and in addition, the flash points of them are low. Accordingly, they are not suitable as electrical insulating oils.
Therefore, as the practical electrical insulating oils, the bicyclic aromatic hydrocarbon having 14 carbon atoms are most preferable among those having not less than 14 carbon atoms.
As described above, it was understood that the bicyclic aromatic hydrocarbons having 14 carbon atoms are most excellent in view of hydrogen gas absorbing capacity and voltage withstanding characteristic.
The number of the bicyclic aromatic hydrocarbons are, however, very large as compared with those having less carbon atoms. For example, they are exemplified by dimethylbiphenyls (symmetrical derivatives and asymmetrical derivatives), ethylbiphenyls, methyldiphenylmethanes, 1,1-diphenylethane and 1,2-diphenylethane; corresponding compounds having an ethylenic double bond such as vinylbiphenyls, 1,1-diphenylethylene and 1,2-diphenylethylene (stilbene). Thus, the number of bicyclic aromatic hydrocarbons having 14 carbon atoms is particularly large as compared with the number of those having 12 or 13 carbon atoms.
It should be noted, however, that all the bicyclic aromatic hydrocarbons having 14 carbon atoms cannot always be used when they are evaluated in view of the practical electrical insulating oil. The reason for this will be described with regard to each group of compounds having 14 carbon atoms.
There are 12 kinds of position isomers of dimethylbiphenyls including symmetrical ones and asymmetrical ones.
As a method for industrially and inexpensively producing dimethylbipheny, only a method of methylation of biphenyl by Friedel-Crafts reaction is known. If it can be directly synthesized from inexpensive alkylbenzenes such as benzene, toluene and xylene, it can be obtained inexpensively. However, the economical direct synthesis is at present impossible.
In the above methylation of biphenyl, methyl groups are often introduced symmetrically due to the orientation of the substituent groups. As a result, a mixture of symmetrical compounds is obtained. For example, 2,2'-dimethylbiphenyl (melting point: +20.degree. C.), 3,3'-dimethylbiphenyl (melting point: +9.degree. C.), and 4,4'-dimethylbiphenyl (melting point: +122.5.degree. C.). All of them have high melting points. Above all, the melting point of rather the main product of 4,4'-dimethylbiphenyl (p,p-isomer) is high.
Accordingly, the dimethylbiphenyls are undesirable for the inexpensive electrical insulating oil because they have too high melting points and inferior in low temperature characteristics.
Among ethylbiphenyls, there are 3 kinds of position isomers, o-ethylbiphenyl, m-ethylbiphenyl and p-ethylbiphenyl. In the industrial production of these ethylbiphenyls, ethylation of biphenyl or transalkylation of ethylbenzene with biphenyl is employed, in which most of the products are m-ethylbiphenyl and p-ethylbiphenyl, while o-ethylbiphenyl is hardly produced in this method.
Accordingly, m-isomer and p-isomer of ethylbiphenyls are practically used.
Methyldiphenylmethanes (benzyltoluenes) are industrially produced and are practically used as electrical insulating oils.
The melting point of 1,1-diphenylethane is as low as -18.degree. C. The melting point of 1,2-diphenylethane is +51.2.degree. C. and the heat of fusion is 5,560 cal/mol, both are high, which are not desirable for improving the low temperature characteristics.
The vinylbiphenyl and other bicyclic aromatic hydrocarbons having 14 carbon atoms and ethylenic double bonds are undesirable as they have polymerization activity due to their double bonds. Furthermore, stilbenes are not suitable because they have high melting points and toxicity toward living bodies.
By the total evaluation as electrical insulating oils on the basis of the foregoing investigation, the following 6 kinds of components (a) to (f) in Table 1 are selected from the bicyclic aromatic hydrocarbons having 14 carbon atoms.
TABLE 1 ______________________________________ Melting Points and Heats of Fusion of Bicyclic Aromatic Hydrocarbons Having 14 Carbon Atoms Melting Point Heat of Fusion Compound (.degree.C.) (cal/mol) ______________________________________ (a) 3-Ethylbiphenyl (m-isomer) -27.6 4000 (b) 4-Ethylbiphenyl (p-isomer) +35.5 2810* (c) o-Benzyltoluene +6.6 5000 (d) m-Benzyltoluene -27.8 4700 (e) p-Benzyltoluene +4.6 4900 (f) 1,1-Diphenylethane -18 4200 Reference Examples 1,2-Diphenylethane +51.2 5560* trans-Stilbene +126 6330* cis-Stilbene +2 3710* 2-Ethylbiphenyl (o-isomer) -6.1 3890 ______________________________________
In Table 2, all the melting points were quoted from published references and the heats of fusion marked with asterisks (*) were actually measured by using Specific Heat Measuring Device, HS-3000 made by Shinku Riko Co., Ltd.
In the above description with regard to the bicyclic aromatic hydrocarbons, the low temperature characteristics were briefly explained. This will be further described in the following passages.
In order to improve the low temperature characteristic of an electrical insulating oil, it is necessary that the composition in which any separation of compounds as crystals does not occur in low temperatures, namely always liquid, must be selected.
The feasibility of crystallizing out relates to viscosities. That is, it is well known that the mobility of liquid molecules is low in a high viscosity liquid and crystals are difficultly separated out even when the temperature is lower than its melting point and such a liquid is liable to become the so-called supercooled state.
The viscosities of all the foregoing 6 components in Table 1 are low under low temperature conditions and the supercooled state rarely occurs. In other words, crystals are liable to be separated out in these components when they are cooled.
There is proposed an electrical insulating oil by retarding the separating out of crystals with positively utilizing the foregoing supercooling phenomenon, in which dibenzyltoluene is mixed into benzyltoluene, one of the foregoing 6 components. This is described in Japanese Patent Publication No. 55-5689 and U.S. Pat. No. 4,523,044 and was commerciallized by Prodelec Co. in France with a trademark "JARYLEC C-100". In the above-mentioned U.S. Pat. No. 4,523,044, the mixture of benzyltoluene-type oligomer and triarylmethane-type oligomer is disclosed. However, the content of triarylmethane-type oligomer is very small in the composition in examples, so that it is apparent that the main components of the composition are benzyltoluene and dibenzyltoluene.
In the above-mentioned composition, however, even when the separating out of crystals is inhibited apparently by the supercooling condition, the viscosity is naturally raised at lower temperatures. Therefore, it is not desirable as an electrical insulating oil, especially as the oil for capacitors.
This fact was confirmed by tracing the disclosure of above-mentioned U.S. Pat. No. 4,523,044.
In the like manner as the example in the above reference, benzylchloride and toluene were reacted by using a catalyst of FeCl.sub.3 ; and benzyltoluene and dibenzyltoluene were obtained by distillation. These benzyltoluene and dibenzyltoluene in a weight ratio of 80:20 were mixed together. The contents of isomers of the benzyltoluene in the obtained mixture were o-isomer: 39.1 wt %, m-isomer: 5.4 wt % and p-isomer: 35.5 wt %, which were almost coincident with the analytical values of the above commercial sample JARYLEC C-100 of o-isomer: 36.2 wt %, m-isomer: 5.9 wt % and p-isomer: 37.8 wt %.
The above synthesized benzyltoluene, benzyltoluene/dibenzyltoluene mixture, and JARYLEC C-100 were respectively put in stoppered test tubes. They were left to stand in a temperature-programmable refrigerator to observe the state of separating out of crystals. One temperature cycle was 12 hours between -40.degree. C. and -50.degree. C.
According to the results of this test, crystals were separated out after 1 to 3 days and the whole was solidified in the case of only benzyltoluene. In the case of the mixture of benzyltoluene/dibenzyltoluene and JARYLEC C-100, the separating out of crystals began after 4 to 7 days and crystals grew gradually, and after 2 weeks, crystals were observed on almost all the walls of test tubes. That is, the viscosity was increased by the addition of dibenzyltoluene to maintain the supercooled state long, and the time period for crystallizing out was prolonged. Accordingly, even though crystals were separated out finally, the crystallizing out was retarded by the addition of dibenzyltoluene.
More particularly, with regard to the isomer mixture of benzyltoluene (o-isomer: 48.9 wt %, m-isomer: 6.8 wt % and p-isomer: 44.3 wt %) obtained in the tracing test of the foregoing U.S. Pat. No. 4,523,044, the quantities of solid phase at several temperatures were calculated in accordance with the following equation of solid-liquid equilibrium, the results of which are shown in FIG. 3.
In the same drawing, the o-isomer is separated out between the points A and B, and the o-isomer and the p-isomer are simultaneously separated out between the points B and C. At point C, the m-isomer participate in them to be separated out together. This point is the eutectic point (-38.9.degree. C.) at which the three components are simultaneously separated out to become a solid. In this drawing, even though the quantity is small, crystals are separated out between -14.degree. C. and -15.degree. C. Accordingly, an isomer mixture of benzyltoluene of the same composition was prepared and it was cooled to a temperature below the eutectic point to change all of them into a solid. After that, the temperature was gradually raised and observed the temperature at which the crystals melted away. The temperature was well coincident with the foregoing temperature within a range of 1.degree. to 2.degree. C.
As described above, benzyltoluene is used with adding dibenzyltoluene in the disclosure of U.S. Pat. No. 4,523,044.
Accordingly, 20% by weight of dibenzyltoluene was added to benzyltoluene. Provided that the dibenzyltoluene is non-crystalline, that is, it is always in a liquid state, as described in the above reference, the relation between the solid-liquid equilibrium and temperatures is in the state as shown in FIG. 4.
According to FIG. 4, the beginning temperature of crystallizing out is lower by about 5.degree. C. as compared with that of FIG. 3. After exceeding -20.degree. C., o-benzyltoluene and p-benzyltoluene begin to separate out. The proportion of solid phase already exceeds 50 wt % at -30.degree. C., 64.5 wt % at -45.degree. C. and 69.3 wt % at -50.degree. C.
In comparison with the foregoing FIG. 3, the composition is not all solid even in the low temperature of -40.degree. C. to -50.degree. C. That is, the composition is apparently improved in view of the existence of the liquid phase. However, in the liquid phase composition with regarding the whole of liquid phase as 100%, the proportion of the dibenzyltoluene is 42% at -30.degree. C., 56% at -40.degree. C. and as much as 65% at -50.degree. C. Thus, in low temperature region, the proportion of the dibenzyltoluene which is unavoidably added in order to lower the melting point exceeds one half quantity in the important liquid phase.
Accordingly, another mixture of benzyltoluene and dibenzyltoluene was prepared so as to correspond to the above liquid phase portion, and the viscosity was measured. As a result, it was understood that the viscosity was too high to be measured at -50.degree. C.
As described above, the crystallizing out can be surely avoided by mixing the dibenzyltoluene, however, this phenomenon is owing to the increase of viscosity. Therefore, it is not desirable.
The above depends upon the solid-liquid equilibrium in which 20 wt % of dibenzyltoluene is mixed. When the quantity of dibenzyltoluene is reduced to a level lower than 20 wt %, the effect to improve the melting point is lowered. On the other hand, when more than 20 wt % is added, even though the melting point is lowered, the viscosity is increased to impair the advantage of the benzyltoluene.
As the condition for an insulating oil having a good low temperature characteristic, especially for the one used in capacitors, the reason for taking notice of the viscosity is as follows:
If there is neither foreign substance nor defect in crystalline structure in solid insulating materials such as film or paper, or there is no weak deteriorated portion of the film caused by an insulating oil, the partial discharge at lower temperatures will firstly occur and the solid insulating materials then suffer damages or the partial discharge expands, thereby the capacitor being broken down.
The conditions until the beginning of partial discharge is considered as follows:
As a preliminary phenomenon, the electric potential is concentrated to the projected portions of electrode or weakened portions of solid insulating material and gases, mainly hydrogen gas, are produced from the insulating oil surrounding the above portions. Gas is produced from one portion at times, or it is produced from plurality of points simultaneously. The produced gas is dissolved in the insulating oil in the initial stage and it is diffused by the difference in gas concentration or the movement of liquid. Meanwhile, because the bicyclic aromatic hydrocarbons generally can absorb hydrogen gas, it is considered that the absorption of gas is occurring in other portions where gas is not produced. When the quantity of produced gas exceeds the quantities to be diffused and absorbed, it exceeds the saturation level and minute bubbles are produced to cause the discharge. One of parameters for controlling this phenomenon is the difficulty in gas generation of an insulating oil, which is considered to be closely related to the hydrogen gas absorbing capacity of the insulating oil. Another parameter is the rate of gas diffusion in the insulating oil. It is considered that the gas diffusion is caused by the combination of the phenomenon of diffusion by the difference in gas concentrations and the phenomenon of transfer of gas by the flow of liquid. Both of these two phenomena are functions of viscosity. If a temperature is the same, it is considered that a lower viscosity is advantageous because the rate of diffusion is large.
With regard to the low temperature characteristic, if the insulating oil must be completely liquid at low temperatures such as -40.degree. C. to -50.degree. C., or in other words, if any solid phase (crystalline phase) must not exist at all, the range of selection of an insulating composition is quite narrow.
In order to discuss the relation between the existence of solid phase and the partial discharge with developing the problem, the following assumption is made. The beginning of crystallizing out occur at many irregular points and crystals gradually grow. When the crystals happen to cover relatively weak portions such as the peripheries of electrode and defective portions of solid insulating material into which electric potential is concentrated, the function of the insulating oil is hindered to cause the occurrence of partial discharge by the load of low electric voltage. With such the assumption, the relation between the lowering of partial discharge owing to the crystallizing out and the quantity of crystals depends upon the probability of the existence of crystals in the relatively weak portions. Accordingly, if a small amount of crystals are separated out, the partial discharge can occur even though its occurrence probability is small. Therefore, it will be accepted that the electrical insulating oil in which the possibility of solid phase to exist is high at low temperatures, is not desirable as an electrical insulating oil for use at low temperatures.
In view of the above, when the foregoing 6 components in Table 1 are considered to be used at a low temperature of -40.degree. C. or -50.degree. C., without saying the use of a single compound, the use of the composition of mixtures of 2 components, or even 4 or 5 components is quite difficult even when the depression of melting points among the components are taken into consideration. For example, when the formulation of an insulating oil can be made with a mixture of 3 or more components in Table 1, the ranges of mixing ratios of the composition are quite narrow, which may not be practically applicable.